Mutations in the adult skeletal muscle sodium channel (hSkM1) have been identified as the cause of a group of rare disorders affecting excitability in human adult skeletal muscle. These disorders are characterized either by episodic muscle weakness or paralysis (hyperkalemic periodic paralysis (HYPP) or by muscle membrane hyperexcitability with sustained myotonic contractions of muscle in response to brief voluntary activity (paramyotonia congenita (PC)). Preliminary studies indicate that these mutations mainly affect the kinetics of sodium channel inactivation. We propose to extend our studies on mutations causing HYPP, PC and other forms of atypical myotonia (AMC) linked to the SkM1 sodium channel gene (SCN4A). These studies will provide important information not only about the pathogenesis of these relatively rare hereditary diseases, but also about the structure and function of the normal sodium channel protein. We will extend our screening studies to identify new SCN4A mutations that cause HYPP, PC, or AMC. We will study the effects of these and currently known mutations on hSkM1 sodium channel kinetics by recreating the mutations in the wild-type hSkM1 background and expressing these mutant-channels in tsA2O1 cells or in the cut-open oocyte preparation. Mutant mechanisms and local protein structure will be assessed further by creating additional mutations at each site or at related sites, and by investigating the interaction of double mutants. Particular attention will be directed to the role of the S4-5 interhelical loop in these disorders as well as to its role in normal channel function. Finally, a limited number of mutations will be introduced into transgenic animals to assess the interaction of the adult muscle environment with the mutant channels, and to investigate the mechanisms by which triggering factors induce abnormal channel behavior.